Multicellular eukaryotic organisms are attacked by numerous parasites from diverse phyla, often simultaneously or sequentially. An outstanding question in these interactions is how hosts integrate signals induced by the attack of different parasites. We used a model system comprised of the plant host Arabidopsis thaliana, the hemibiotrophic bacterial phytopathogen Pseudomonas syringae, and herbivorous larvae of the moth Trichoplusia ni (cabbage looper) to characterize mechanisms involved in systemic-induced susceptibility (SIS) to T. ni herbivory caused by prior infection by virulent P. syringae. We uncovered a complex multilayered induction mechanism for SIS to herbivory. In this mechanism, antiherbivore defenses that depend on signaling via (1) the jasmonic acid-isoleucine conjugate (JA-Ile) and (2) other octadecanoids are suppressed by microbe-associated molecular pattern-triggered salicylic acid (SA) signaling and infection-triggered ethylene signaling, respectively. SIS to herbivory is, in turn, counteracted by a combination of the bacterial JA-Ile mimic coronatine and type III virulence-associated effectors. Our results show that SIS to herbivory involves more than antagonistic signaling between SA and JA-Ile and provide insight into the unexpectedly complex mechanisms behind a seemingly simple trade-off in plant defense against multiple enemies.
Herbivorous insects are among the most successful radiations of life. However, we know little about the processes underpinning the evolution of herbivory. We examined the evolution of herbivory in the fly, Scaptomyza flava, whose larvae are leaf miners on species of Brassicaceae, including the widely studied reference plant, Arabidopsis thaliana (Arabidopsis). Scaptomyza flava is phylogenetically nested within the paraphyletic genus Drosophila, and the whole genome sequences available for 12 species of Drosophila facilitated phylogenetic analysis and assembly of a transcriptome for S. flava. A time-calibrated phylogeny indicated that leaf mining in Scaptomyza evolved between 6 and 16 million years ago. Feeding assays showed that biosynthesis of glucosinolates, the major class of antiherbivore chemical defense compounds in mustard leaves, was upregulated by S. flava larval feeding. The presence of glucosinolates in wild-type (WT) Arabidopsis plants reduced S. flava larval weight gain and increased egg-adult development time relative to flies reared in glucosinolate knockout (GKO) plants. An analysis of gene expression differences in 5-day-old larvae reared on WT versus GKO plants showed a total of 341 transcripts that were differentially regulated by glucosinolate uptake in larval S. flava. Of these, approximately a third corresponded to homologs of Drosophila melanogaster genes associated with starvation, dietary toxin-, heat-, oxidation-, and aging-related stress. The upregulated transcripts exhibited elevated rates of protein evolution compared with unregulated transcripts. The remaining differentially regulated transcripts also contained a higher proportion of novel genes than the unregulated transcripts. Thus, the transition to herbivory in Scaptomyza appears to be coupled with the evolution of novel genes and the co-option of conserved stress-related genes.
Experimental infections of Arabidopsis thaliana (Arabidopsis) with genomically characterized plant pathogens such as Pseudomonas syringae have facilitated the dissection of canonical eukaryotic defence pathways and parasite virulence factors. Plants are also attacked by herbivorous insects, and the development of an ecologically relevant genetic model herbivore that feeds on Arabidopsis will enable the parallel dissection of host defence and reciprocal resistance pathways such as those involved in xenobiotic metabolism. An ideal candidate is Scaptomyza flava, a drosophilid fly whose leafmining larvae are true herbivores that can be found in nature feeding on Arabidopsis and other crucifers. Here, we describe the life cycle of S. flava on Arabidopsis and use multiple approaches to characterize the response of Arabidopsis to S. flava attack. Oviposition choice tests and growth performance assays on different Arabidopsis ecotypes, defence-related mutants, and hormone and chitin-treated plants revealed significant differences in host preference and variation in larval performance across Arabidopsis accessions. The jasmonate and glucosinolate pathways in Arabidopsis are important in mediating quantitative resistance against S. flava, and priming with jasmonate or chitin resulted in increased resistance. Expression of xenobiotic detoxification genes was reduced in S. flava larvae reared on Arabidopsis jasmonate signalling mutants and increased in plants pretreated with chitin. These results and future research directions are discussed in the context of developing a genetic model system to analyse insect-plant interactions.
Insects use chemical cues to identify host plants, which suggests that chemosensory perception could be a target of natural selection during host specialization. Five papers using data from the 12 recently sequenced Drosophila genomes examined chemosensory gene function and evolution across specialist and generalist species. A functional study identifies odorant binding proteins that mediate loss of toxin avoidance in a specialist, and targeted genomic studies indicate specialists and island endemics lose chemosensory genes more rapidly than generalist and mainland relatives. Together, these studies suggest a mode of chemoreceptor evolution dominated by birth/death dynamics, coupled with a low level of potential positive selection.
Many pathogens are virulent because they specifically interfere with host defense responses and therefore can proliferate. Here, we report that virulent strains of the bacterial phytopathogen Pseudomonas syringae induce systemic susceptibility to secondary A syringae infection in the host plant Arabidopsis thaliana. This systemic induced susceptibility (SIS) is in direct contrast to the well studied avirulence/R gene-dependent resistance response known as the hypersensitive response that elicits systemic acquired resistance. We show that A syringae-elicited SIS is caused by the production of coronatine (COR), a pathogen -derived functional and structural mimic of the phytohormone jasmonic acid (JA). These data suggest that SIS may be a consequence of the previously described mutually antagonistic interaction between the salicylic acid and JA signaling pathways. Virulent P. syringae also has the potential to induce net systemic susceptibility to herbivory by an insect (Trichoplusia ni, cabbage looper), but this susceptibility is not caused by COR. Rather, consistent with its role as a A mimic, COR induces systemic resistance to T. ni. These data highlight the complexity of defense signaling interactions among plants, pathogens, and herbivores.
Plants have evolved different but interconnected strategies to defend themselves against herbivorous insects and microbial pathogens. We used an Arabidopsis/Pseudomonas syringae pathosystem to investigate the impact of pathogen-induced defense responses on cabbage looper (Trichoplusia ni) larval feeding. Arabidopsis mutants [npr1, pad4, eds5, and sid2(eds16)] or transgenic plants (nahG) that are more susceptible to microbial pathogens and are compromised in salicylic acid (SA)-dependent defense responses exhibited reduced levels of feeding by T. ni compared with wild-type plants. Consistent with these results, Arabidopsis mutants that are more resistant to microbial pathogens and have elevated levels of SA (cpr1 and cpr6) exhibited enhanced levels of T. ni feeding. These experiments suggested an inverse relationship between an active SA defense pathway and insect feeding. In contrast to these results, there was increased resistance to T. ni in wild-type Arabidopsis ecotype Columbia plants that were infected with P. syringae pv. maculicola strain ES4326 (Psm ES4326) expressing the avirulence genes avrRpt2 or avrB, which elicit a hypersensitive response, high levels of SA accumulation, and systemic acquired resistance to bacterial infection. Similar results were obtained with other ecotypes, including Landsberg erecta, Cape Verdi Islands, and Shakdara. When infected with Psm ES4326(avrRpt2) or Psm ES4326(avrB), nahG transgenic and npr1 mutant plants (which are more susceptible to virulent and avirulent P. syringae strains) failed to show the increased insect resistance exhibited by wild-type plants. It was surprising that wild-type plants, as well as nahG and npr1 plants, infected with Psm ES4326 not expressing avrRpt2 or avrB, which elicits disease, became more susceptible to T. ni. Our results suggest two potentially novel systemic signaling pathways: a systemic response elicited by HR that leads to enhanced T. ni resistance and overrides the SA-mediated increase in T. ni susceptibility, and a SA-independent systemic response induced by virulent pathogens that leads to enhanced susceptibility to T. ni.
The generalist insect herbivore Trichoplusia ni (cabbage looper) readily consumes Arabidopsis and can complete its entire life cycle on this plant. Natural isolates (ecotypes) of Arabidopsis are not equally susceptible to T. ni feeding. While some are hardly touched by T. ni, others are eaten completely to the ground. Comparison of two commonly studied Arabidopsis ecotypes in choice experiments showed that Columbia is considerably more resistant than Landsberg erecta. In no-choice experiments, where larvae were confined on one or the other ecotype, weight gain was more rapid on Landsberg erects than on Columbia. Genetic mapping of this difference in insect susceptibility using recombinant inbred lines resulted in the discovery of the TASTY locus near 85 cM on chromosome 1 of Arabidopsis. The resistant allele of this locus is in the Columbia ecotype, and an F, hybrid has a sensitive phenotype that is similar to that of Landsberg erecta. The TASTY locus is distinct from known genetic differences between Columbia and Landsberg erecta that affect glucosinolate content, trichome density, disease resistance, and flowering time.